The purpose of this study is to determine the biochemical mechanisms by which cholinergic stimulation reduces the slow Ca2 ion current in atrial and ventricular myocardium. The rationale for this study is based on evidence that beta-adrenergic receptor stimulation increases the slow Ca2 ion current because of activation of cAMP-facilitated membrane phosphorylation. Muscarinic receptor stimulation is known to attenuate catecholamine induced increases in intracellular cAMP and, independently, it has been shown that muscarinic receptor activation decreases the magnitude of the slow Ca2 ion current. The hypothesis that will be tested in this study is that muscarinic receptor stimulation diminishes the slow Ca2 ion current because it directly diminishes the activity of the enzyme adenylate cyclase. Thus under conditions of catecholamine stimulation of adenylate cyclase and augmentation of the slow Ca2 ion current, muscarinic receptor stimulation decreases the activity of adenylate cyclase resulting in attenuation of intracellular cAMP levels thus effectively reducing the number of slow Ca2 ion channels phosphorylated by cAMP directed mechanisms. In order to systematically test this hypothesis as well as alternative mechanisms, the biochemical (cAMP, cGMP, cAMP-protein kinase) and electrophysiologic (excitably threshold) effects of acetylcholine will be determined on calcium-dependent action potentials (tetrodotoxin blockade of fast Na ion channels) produced by: 1) increasing extracellular calcium (to test if acetylcholine directly blocks slow Ca2 ion channels), 2) alpha-adrenergic receptor stimulation (to test if acetylcholine blocks catecholamine induced changes in slow Ca2 ion current independent of cAMP directed mechanisms), 3) beta-adrenergic stimulation (to test if acetylcholine only affects the slow Ca2 ion current in the presence of beta-adrenergic receptor activation), and 4) increasing intracellular cAMP content independent of catecholamines (to test if acetylcholine decreases the slow Ca2 ion current directly through its effects on the cAMP system). In a separate series of experiments, cGMP will be directly elevated independent of muscarinic receptor activation, to determine if this intervention mimics the effects of acetylcholine.